How to Use a Logic Probe

- details of how to use a logic probe and some of the tips about using logic probes

In this section

A logic probe can be used to provide an indication about the signals on logic lines in digital circuits.

A logic probe is a simple items of electronic test equipment and as a result, using a logic probe is quite easy, however its limitations should be remembered when using it test circuits.

Basics of using a logic probe

Understanding how to use a logic probe is very straightforward. There are many different logic probe products, all of which are slightly different, but they all conform to the same basic overview of their functionality and operation, for example some have audible indications as well as the visual ones whereas others may not.

Accordingly it is possible to give some basic guidelines about how to use a logic probe, but there will be slight variation in the way they are used dependent upon the particular logic probe in use.

Logic probe connections and controls

Logic probe connections

Before using the logic probe, it is necessary to understand the connections.

As can be seen, there are three connections to the logic probe:

Black lead with crocodile clip: There are two leads which generally come out of the opposite end of the instrument to the metal probe itself. The black lead is connected to negative ground and is also used as the return.

Red lead with crocodile clip: This lead will have red on it somewhere, possibly just on the crocodile / alligator clip and is used to connect to the supply. Be careful to connect this to the logic supply which will normally be +5 volts. Read the instructions to see over what range the probe will operate.

Probe: The probe, as shown on the diagram is a metal point used for probing the circuit.

The first requirement before using the logic probe is to connect the power connections to the circuit. Apart from ensuring they are the right voltage, the points used for connecting the crocodile clips to should be accessible and provide a reliable connection without the risk of touching any adjacent components or other connections.

Note: If possible connect the power connections to the unit under test when it is powered off. In this way risk of damage due to shorting, etc. is minimised.

Initial settings

When using the logic probe, it is necessary to select the required settings on the switches. Dependent upon the logic probe manufacturer and model, there are a number of options that may need to be set:

TTL / CMOS: It is necessary to select the logic family. Normally two options are given, namely CMOS and TTL. As the high and low states of these two logic families are slightly different, it is necessary to select the correct option.

MEM / PULSE : This is used to select he operational mode of the logic probe. The Pulse position is used for normal operation for pulse or level detection. The MEM or memory position is used to capture a pulse. For example if it is necessary to detect whether a pulse has occurred or not.

Note: Some surface mount devices these days use supply rails of 3.3 volts or less. Most logic probes will not work with these ICs as the logic levels cannot normally be accommodated. Additionally it is often difficult to probe surface mount boards as there is a real danger of shorting pins.

Probing & results

With power applied to the circuit under test and the logic probe, it is possible to use it to probe various points on the circuit.

One easy point to locate may be a transistor driver. The can of the transistor is often connected to its collector making a point where the signal can be easily accessed.

The logic probe will indicate which lines are high, low or carrying a signal.

It is then a matter of interpreting the results in line with the circuit to find out whether they are acting correctly or not.

A brief order of using a logic probe could be:

Connect the black clip or line to ground or to a common line of the circuit to be tested. This assumes that 0V and ground / common are the same.

Secondly connect the red clip or leave to the positive supply of the circuit.